Air is typically separated in a dual pressure (double column based) distillation system. In this process, air is compressed to an elevated pressure (5.5+ bara), pre-purified, cooled and directed to a moderate pressure nitrogen rectification section where the feed air is distilled into a nitrogen rich vapor/liquid overhead and an oxygen enriched bottom liquid (kettle). These enriched liquid streams are subsequently subcooled, depressurized and fed to a low pressure (near ambient) distillation system where the oxygen rich kettle liquid is further fractionated into an essentially pure oxygen bottoms product and a further enriched nitrogen overhead product(s) and/or waste stream.
Argon constitutes a minor portion of ambient air (0.93%). However, it possesses a high unit value. As a consequence, its recovery from the base double column system is often desirable. Argon can be recovered from the double column system by extracting an argon rich draw from the upper column near the base of the nitrogen stripping section. The argon rich stream is then directed to an argon rectification section where argon may be produced overhead. The overhead condensation load is typically imparted to at least a portion of the oxygen rich column bottoms stream (prior to introduction into the primary low pressure distillation column). Argon can be produced directly by “superstaged” distillation to merchant liquid purities (˜1 ppm O2, ˜180 stages) or to intermediary purities (1 to 2%, ˜50 stages) and subsequently refined by catalytic oxidation (typically employing hydrogen).
Modern air separation plants almost exclusively employ superstaged distillation for high purity argon production. However, such systems will typically attain column/coldbox heights in excess of 200+ ft. As a consequence, considerable expense is incurred to attain high purity (split columns, multiple coldbox sections, liquid reflux/return pumps). This situation is further compounded for large air separation plants where column feed/draw re-distribution points typically consume more height. There exists a need to drastically shorten the argon distillation column without returning to the use of catalytic combustion (and its associated complexity and operating costs). The subject invention targets the economically weakest portion of the argon-oxygen distillation. The upper half of the argon column serves to remove less than 1% of the oxygen contained in argon column feed. Since distillation cost is proportional to the logarithm of purity a substantial cost (and height) is incurred in attaining 1 ppm O2 in argon.
There have been integrations within the prior art between air separation plants and adsorbents with the intent to eliminate expensive post conditioning systems to remove oxygen and nitrogen from the argon. For instance, in U.S. Pat. No. 4,717,406 a liquid phase adsorption process wherein in a feed from a cryogenic plant is directed to an adsorption system. The adsorption system serves to purify the liquefied gas prior to introducing it into a liquid storage tank. The targeted applications include the removal of water and CO2 from electronics grade gases (e.g. LO2). The regeneration method disclosed is temperature swing. U.S. Pat. No. 5,685,172 details a process targeting the removal of trace oxygen (and CO) from a variety of inert gases. The process also notes direct liquid processing and argon is cited as an example fluid. Metal oxides (CuO, MnO2) are detailed as adsorbents for oxygen. Regeneration is accomplished through the use of a reducing gas (H2) at modest temperature (150˜250 C). The use of a reducing gas makes it difficult to integrate the adsorbent beds with the air separation plants in that the reducing gas is not made in the air separation plant, but must be on hand to regenerate the adsorbents. More importantly, during regeneration of the adsorbent, argon rich fluids will be lost from the process. U.S. Pat. No. 7,501,009 details a cyclic adsorption process for the purification of argon. The process may be operated at cryogenic temperature while processing crude argon in the gaseous state. Zeolites are noted as possible adsorbents for a pressure swing adsorption (PSA). Regeneration gas is directed back to the argon-oxygen rectification column. The problem with this type of integration is that it requires the inclusion of a crude argon compressor (and associated power consumption). Lastly, U.S. Pat. No. 5,601,634 discloses an integration in which both nitrogen and oxygen contained in the argon are removed in adsorbent beds. The problem with this type of integration is that the vapor must be re-liquefied resulting in increased power consumption.
As will be discussed, among other advantages, the present invention provides an integration in which an argon rich liquid stream produced through separation of the argon from air in an air separation plant is purified with an adsorbent to allow the purified liquid to be conventionally stored and that can easily be integrated with an air separation plant.